Epidemiologic data suggest that vitamin D may play a role in the pathogenesis of prostate cancer. Moreover, in vitro and animal studies have shown that vitamin D inhibits the growth of prostate cancer cells. This study is designed to examine the potential clinical utility of vitamin D in treating men with early stages of prostate cancer. Men are enrolled into this study if, after primary treatment of their prostate cancer (surgical prostatectomy or radiation therapy), they demonstrate a rising serum level of the prostate tumor marker, Prostatic Specific Antigen (PSA). They must have had stage A or B disease and not have evidence of metastatis, and therefore have minimal recurrent disease. The primary endpoint of the study is to show a slowing of the rate of rise of PSA levels as an index of slowed prostate cancer growth while on Vitamin D therapy (calcitriol, Rocaltrol- Roche). Eight otherwise healthy men have thus far been enrolled in this study. One subject was excluded prior to the initiation of therapy due to the identification of potential metasatic disease. Seven subjects have been initiated on calcitriol therapy. One subject was withdrawn from therapy due to the development of a kidney stone. Three subjects have shown a beneficial response to the therapy, with a slowing of the PSA rate of rise compared to the rate of rise before therapy was initiated. The study is ongoing, and recruitment of patients is continuing.